This invention relates to a fuel gas steam reformer assemblage which is formed from a plurality of repeating sub-assemblies. More particularly, this invention relates to a fuel gas steam reformer assemblage which is compact and lighter in weight than conventional steam reformer assemblages used in fuel cell power plants and which operates at lower burner gas passage wall temperatures.
Fuel cell power plants include fuel gas steam reformers which are operable to catalytically convert a fuel gas, such as natural gas, into a gas stream containing primarily hydrogen and carbon dioxide. The conversion involves passing a mixture of the fuel gas and steam through a catalytic bed which is heated to a reforming temperature of about 900xc2x0 F. to about 1,600xc2x0 F. The difference between the bulk reactant gas and the reactor walls can be as much as 400xc2x0 F. to 500xc2x0 F. Catalysts typically used are nickel catalysts which are deposited on alumina pellets. A typical reformer will consist of a plurality of reaction tubes which are contained in a housing that is insulated for heat retention. The reaction tubes are heated by burning excess fuel gas in the housing and passing the burner gasses over the reaction tubes. The individual reaction tubes will typically include a central exhaust passage surrounded by an annular entry passage. The entry passage is filled with the catalyzed alumina pellets, and a fuel gas-steam manifold is operable to deliver the fuel gas-steam mixture to the bottom of each of the entry passages whereupon the fuel gas-steam mixture flows through the catalyst beds. The resultant heated hydrogen-rich gas stream then flows through the central exhaust passages in each tube so as to assist in heating the inner portions of each of the annular catalyst beds; and thence from the reformer for further processing and utilization.
Steam reformers require a large amount of surface area in the catalyst bed in order to provide a high degree of catalyst-fuel mixture interaction and a large heat transfer surface area to produce the amount of hydrogen required to operate the fuel cells at peak efficiency. This need for large catalyst bed and heat transfer surface area, when met by using catalyst-coated pellets in tubular reformers, results in undesirably large and heavy reformer assemblies. For example, a commercially available 200 KW acid fuel cell power plant includes a steam reformer component which has a volume of about 150 to 175 cubic feet; and weighs about 3,500 lbs. It would be highly desirable to provide a steam reformer which is suitable for use in a fuel cell power plant, which reformer supplies the necessary surface areas, but is compact and light in weight.
U.S. Pat. No. 5,733,347 granted Mar. 31, 1998 to R. R. Lesieur describes a compact light weight fuel gas reformer which has adjacent burner gas and fuel gas passages. The burner and fuel gas passages both contain heat transfer fins which extend for the entire length of the passages. When the structure shown in the aforesaid patent is operated at 1,500xc2x0 F. + temperatures with its counter flow arrangement of burner gas and process gas, undesirably high burner passage wall temperatures may occur due to high heat transfer through the fins toward the front and middle portions of the burner gas passages, which high heat transfer is not offset by a high rate of heat absorption in the process gas stream. The wall temperatures will drop after the middle of the burner gas passages because of the higher heat absorption by the incoming process gas, however the excessive heat produced may damage the reformer walls.
It would be desirable to provide a compact, light weight reformer assemblage such as that shown in the aforesaid patent, but which would not be subjected to the over heating problem described above.
This invention relates to a steam reformer structure which provides the necessary catalyzed and heat transfer surface area, is substantially smaller and lighter than conventional steam reformers, and can be operated at lower wall temperatures. The steam reformer structure of this invention is formed from a series of essentially flat plate reformer components. Each of the reformer components includes outer reformer passages sandwiched around a plurality of central regenerator/heat exchanger passages.
The basic configuration of the catalyzed reformer consists of repeating modules of burner-reformer-regenerator-reformer-burner elements. Gases in the burner and regenerator sections flow in one direction, and the fuel gases flow through the reformer passages in the opposite direction. This is a desirable configuration of a reformer where the reformed gas passes counter flow to the reformer reactant stream so that it can transfer its heat to the reactant stream.
Heat is generated by the combustion of burner gases as they pass through the burner cavity and burn catalytically on the burner walls. The reforming reaction is very endothermic and because of the dynamic equilibrium also requires high temperatures to achieve high conversion. With a high catalytic surface area in the burner, the burner gas combustion would take place at the top of the reactor. However, at this location, most of the reforming has already taken place, so there would be insufficient cooling from the reformer endothermic reaction to keep the walls"" temperature within a reasonable range, i.e., below about 1,600xc2x0 F. To maintain wall temperature within the acceptable range at the inlet end of the burner gas passages, heat transfer fins are eliminated in this section. The system configuration which eliminates high burner wall temperatures consists of an inlet section having no heat transfer fins; and intermediate section having a low density of heat transfer fins; and an outlet section having a higher density of heat transfer fins. By fin xe2x80x9cdensityxe2x80x9d, I am referring to the number of fins per inch of burner passage taken in the direction which is transverse to the direction of burner gas flow. The wall temperature will still spike, but it will be maintained at levels which are lower than 1,600xc2x0 F. All other metal temperatures are less than 1,500xc2x0 F. It is noted that with a proper system configuration, the temperature can be manipulated so that the walls remain relatively cool for longer useful life. A high fin density is maintained throughout the entirety of both the reformer and the regenerator passages so as to force the reformer gas to come very close to the reformer wall temperatures, hence providing fuel conversion on the reformer side and low regenerator (process) exit temperatures.
Based on the reformer model, a conceptual design of a 200 kW. reformer (7,000 ft3 of H2/hr) was made. It consisted of ten elements with each element having a burner, two reformer sections and a regenerator section. The burner gas was vented to the bottom of the reformer, the process gas exhaust to the left, and the process gas inlet to the right. This type of configuration simplifies all of the manifolding. The total thickness of each element was about 1.5 inches. Axial waviness of the fins adds to the strength of the elements as well as increasing the surface area and heat transfer characteristics by enhancing gas flow turbulence. The first 40% of the burner gas passages was devoid of heat transfer fins; the next 10% of the burner gas passages was provided with a low density fin structure, i.,e., approximately one fin per inch; and the remaining 50% of the burner gas passages was provided with a higher density fin structure, i.,e., approximately ten fins per inch.
Using the aforesaid fin density arrangement, peak reformer gas temperatures of about 1,500xc2x0 F. were obtained, while peak burner wall temperatures were kept at about 1,600xc2x0 F. This compares with comparable reformer gas temperatures achieved with the prior art structure and burner gas passage wall temperatures of above 1,700xc2x0 F.
It is therefore an object of this invention to provide an improved fuel gas steam reformer assembly for use in a fuel cell power plant.
It is a further object of this invention to provide a reformer assembly of the character described which has a longer useful life due to temperature control of burner gas passage walls of the assembly.
It is another object of this invention to provide a reformer assembly of the character described wherein the highest wall temperature experienced in the assembly is less than about 1,600xc2x0 F.
It is yet another object of this invention to provide a reformer assembly of the character described which employs serpentine gas flow and heat transfer fins in the gas passages, which fins vary in population density in different areas of the assembly.